Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. The rotor blades are the primary elements for converting wind energy into electrical energy. The blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between its sides. Consequently, a lift force, which is directed from the pressure side towards the suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is connected to a generator for producing electricity.
Typically, wind turbines are designed to operate at a rated power output over a predetermined or anticipated operating life. For instance, a typical wind turbine is designed for a 20-year life. However, in many instances, this anticipated operating life is limited or based on the anticipated fatigue life of one or more of the wind turbine components. For instance, FIG. 1 illustrates example data for a conventional wind turbine having an anticipated fatigue life (the end of such time period being indicated by line 50). As shown, the projected calculated cumulative fatigue damage is represented by line 52, whereas the actual cumulative fatigue damage is represented by line 54. The projected cumulative fatigue damage 52 may be linear or non-linear with respect to time depending upon the component and maintenance practices. For example, as shown, the projected cumulative fatigue damage 52 is linear from time T0 to T1 and T3 to T4, whereas the projected cumulative fatigue damage 52 is substantially non-linear from T1 to T3. The non-linear portions may be caused from a variety of varying wind conditions and/or operation curtailment. For example, the projected cumulative fatigue damage 52 between T1 to T2 may have been caused by a wind boost or a power up, whereas the projected cumulative fatigue damage 52 between T2 to T3 may have been curtailed by an operator or grid.
The actual cumulative fatigue damage is typically less than the projected cumulative fatigue damage due, at least in part, to the following: (1) conservative design assessment of the site wind conditions (e.g. mean wind speed and effective turbulence intensity), (2) non-operating hours that were not considered as part of the site assessment, and (3) experience gained over time of the true load carrying capabilities of the component material and structure. It should be understood that the terms “turbulence intensity,” “turbulence,” “turbulence intensity level” and/or similar terms may be used herein interchangeably.
The difference between the projected cumulative fatigue damage 52 and the actual cumulative fatigue damage 54 is illustrated by space 55. As such, knowing the accurate fatigue life consumption allows an operator to determine a remaining economic value or fatigue reserve of the wind turbine at any point in time. Thus, if the actual cumulative fatigue damage 54 is less than the projected cumulative fatigue damage 52, as indicated by the space 55 of FIG. 1, the operator can operate the wind turbine beyond the design life 50 depending on the estimated fatigue life consumption. For example, as shown in FIG. 1, the remaining fatigue life reserve at the end of the life of the wind turbine is represented by dimension 56. Similarly, and referring now to FIG. 2, if the actual cumulative fatigue damage 54 is greater than the projected cumulative fatigue damage 52 (as indicated by space 55), then the operator can operate the wind turbine less than the design life 50 (as indicated by line 57 at time T1) or in a manner to achieve the design life 50, depending on the estimated fatigue life consumption.
The fatigue life consumption of the wind turbine as used herein is the life of the wind turbine that has been consumed or exhausted by previous operation. The fatigue life consumption should be determined not only by the number of elapsed years or cycles since the wind turbine has been in operation, but also by the cumulative operating hours of the wind turbine at different turbine operating states. The terms “operating hours,” “available hours,” or similar are meant to encompass the hours spent in any of the various operational modes of the wind turbine, whereas the terms “non-operating hours,” or “non-available hours” are the hours where the wind turbine is shut down (e.g. due to servicing, replacement of components, scheduled shut-downs, etc.). Further, the term “idling hours” is meant to encompass the operating and/or available hours that the wind turbine operates at wind speeds below cut-in wind speeds or when the unit is curtailed by the operator.
Conventional wind turbines have implemented various control technologies to estimate the fatigue life consumption and therefore curtail fatigue loads. For example, various systems utilize a plurality of sensors to detect loads acting on the wind turbine and limits turbine operation accordingly. More specifically, for certain wind conditions, the system may decrease the output power level of the wind turbine for certain wind speeds to reduce the loads acting on the wind turbine. Another system simply shuts down the wind turbine when wind speeds exceed a certain value.
In addition, European Patent Application EP 2 302 207 entitled “Power Generating Machine Load Control Based on Consumed Fatigue Life Time and Real-Time of Operation of a Structural Component” dated Sep. 23, 2009 discloses a control technology that estimates consumed fatigue life time by detecting a load cycle of a wind turbine component. More specifically, the method determines a consumed fatigue life time of a wind turbine component based on the load cycles of the component and a real-time of operation of the component (i.e. how long the turbine has been in commission), compared to the fatigue life time with the real-time of operation, and controls the wind turbine accordingly.
Such control technologies, however, are inefficient at accurately and reliably predicting the fatigue life consumption. More specifically, previous systems and methods do not consider cumulative operating hours of the wind turbine at different wind turbine operating states (e.g. power levels) when estimating fatigue life consumption. As such, current control strategies may lead to a sub-optimal Net-Present-Value (NPV) of the wind turbine when compared to a wind turbine where fatigue curtailment can be postponed until a later date (i.e. the revenue stream is not lowered as quickly). Further, the sensors associated with conventional control systems add additional costs and may fail during operation of the wind turbine, thereby providing unreliable data for determining actual fatigue life consumption.
Accordingly, an improved system and method for operating a wind turbine based on fatigue life consumption would be desired in the art. For instance, a system and method for operating a wind turbine that estimates the fatigue life consumption based on cumulative operating hours at different power levels combined with wind direction would be advantageous.